The present invention concerns an improvement for continuous thin-bodies casters. More precisely it concerns a system capable of maximizing plant productivity and product yield, by eliminating wear on plates and rolls caused by their mutual contact, thus reducing plant stoppages for removal of metal solidified between plates and rolls and minimizing the occurrence of shape defects on the edges of the casting such as flashes, profile variations, etc.
One of the technologies for the continuous casting of thin products provides for the casting of liquid metal, steel for instance, in a mold bounded by a pair of counter-rotating rolls with parallel axes lying in the same horizontal plane, and separated by a distance greater than the sum of the radii of the rolls and by two plates positioned at the flat ends of the rolls.
The plates consist essentially of refractory materials backed by a metal frame and in contact with the flat end faces of the rolls and the liquid metal. Thrust devices, springs for example, keep the plates hard against the roll contact surfaces to ensure the seal needed to contain the liquid metal.
This seal is subject to particularly difficult operating conditions because in addition to the thermal stress that is always present, there is continuous sliding contact between the plates and the flat ends of the rolls. This causes wear of the contact surfaces which allows the infiltration of steel.
The flashes that form the edges of the casting have a markedly adverse effect on surface quality in that part of the product and the yield of the heat and may even jam the rolls.
Regarding the abrasion problem, efforts have been made to optimize plate construction and also to increase the back-up pressure, while ensuring a flush fit with the rolls.
For instance, there have been proposals for composite plates having insulating materials in the central part to limit steel solidification, and wear-resistant seal materials in the zone in contact with the rolls. In this case, in addition to the problems of assembly and dimensional tolerances, there are big difficulties of the joints between insulating material and seal material, since these form a preferential point of anchorage for undesired fragments of solidified metal.
Monolithic plates made of abrasion-resistant refractories such as alumina or silica have also been proposed.
Where roll life is concerned, stellite facings have been used on the parts in contact with the plates and subject to abrasion due to reciprocal sliding.
However, none of these measures has completely resolved the difficulties described. Although attenuated, such difficulties as plate/roll seal, surface-quality of castings and roll jamming, still persist.
Attempts have also been made to utilize electromagnetic arrangements to resolve the problem of confinement of liquid metal, such as steel for instance, at the flat ends of the rolls; for example, in U.S. Pat. No. 4,936,374, the containment plates are replaced by a ac-fed electromagnetic device which produces a force in a direction such as to contain the steel. However, there are some drawbacks with this solution too. In fact, owing to the high density of the steel (about 7.2 kg/dm3) large, complex, high powered devices are needed to contain electromagnetically all the liquid metal present in the mold. There is also the possibility of stability problems with the induction generated field of force with ensuing loss of containment efficiency or at least lack of uniformity on the edges of the cast product. Hence such solutions appear to be economically and technically unacceptable.
Regarding the quality obtained, especially the problem of surface defects and shape near the corners of the cast products, attention must be focused on the importance of obtaining a product having a solidification profile that is as even and uniform as possible along the periphery of the cast section, so as to prevent the formation of weak points (cracks) and also avoid the need for costly operations downstream to remove the defects.
Attempts have been made in the past to eliminate these portions of defective products by grinding or trimming the edges of the strip or thin plate, for instance. However, such operations have proved unsatisfactory because of the cost and the space requirement of the equipment necessary and because of the volume of materials to be rejected or treated as scrap.
It is important to stress the difference which exists in the yield in the case of thin flat products on the one hand and thicker products on the other. In fact, since a certain quantity of material has to be eliminated, the percentage of waste on a thicker product is low, but where a thin product is concerned the proportion increases, of course, and can reach unacceptable levels.
Hence, notwithstanding the progress made so far in the continuous casting of thin products, the problems of confinement of a liquid metal in the mold, the surface quality of the casting at the edges and the jamming of the caster due to flashes of metal solidified between containment plates and rolls still remain unsolved.